This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Japan Patent Application No.11-307629 filed in Japan on Oct. 28, 1999, the entire content of which is herein incorporated by references.
1. Technical Field
The present invention relates to a machine for continuous casting of steel and a method for continuous casting of steel, by which the occurrence of periodical molten steel level fluctuations in a mold, as caused by slab bulging can be prevented and good-quality slabs, which have no surface defects due to mold powder trapping and no longitudinal surface cracks, can be obtained.
2. Background Art
In continuous casting, a slab is supported and guided by a plurality of guide roll pairs and withdrawn by pinch roll pairs. However a slab between the guide roll pairs is not supported, therefore, a bulging occurs in the slab which contains a liquid core. When the bulged portion of the slab passes through a next downstream guide roll pair, the portion having an increased thickness due to bulging undergoes reduction by the paired guide rolls.
When the slab bulges, some portion of liquid core molten steel in the upstream side relative to the bulged portion flows into the bulged portion and, when the bulged portion undergoes reduction by rolls, some portion of molten steel in the bulged portion is pressed back to the upstream side accordingly. When the amount of liquid core molten steel flowing from the upstream side into the bulged portion and the amount of liquid core molten steel subsequently pressed back from the bulged portion to the upstream side are constant with the lapse of time, no periodic molten steel level fluctuations occur in the mold.
However, the slab solidification is uneven in the direction of casting as well as in the direction of slab width, and the bulging-caused flow of molten steel is also uneven. As a result, the extent of bulging varies not only in the direction of the casting but also varies with the lapse of time even at the same place. Therefore, the amount of liquid core molten steel flowing from the upstream side into the bulged portion and the amount of molten steel pressed back from the bulged portion to the upstream side are not always equal upon each repetition of slab bulging and reduction of the bulged portion, so that periodical molten steel level fluctuations, which are up and down fluctuations of the molten metal level within the mold, occur.
Further, when, in guide roll pairs, the roll-to-roll distances in the direction of casting are the same, but the periodical molten steel level fluctuations within the mold are enhanced and become greater. The extent, per occurrence, of bulging occurring between two neighboring rolls is generally several hundred micrometers and, therefore, even when the bulged portion returns to the original thickness before bulging as a result of reduction by the next downstream side rolls, the fluctuations in the amount of liquid core molten steel flowing into from the upstream side and pressed back are small, hence the periodic molten steel level fluctuations within the mold are slight. However, in cases where the roll-to-roll distances for many guide roll pairs are the same, the molten steel level fluctuations due to bulging become periodic and great fluctuations. The periodical molten steel level fluctuations due to ordinary bulging generally amount to about xc2x12.5 to xc2x110 mm vertically at the reference level.
Where the magnitude of periodical molten steel level fluctuations is great, the mold powder may be entrapped in the molten steel in the mold, causing the occurrence of defects on the slab surface, due to mold powder trapping, or longitudinal cracks on the slab surface. Furthermore, when the periodic molten steel level fluctuations is particularly great, the operation may become difficult to continue.
Concerning the prevention of such periodic molten steel level fluctuations within mold as resulting from slab bulging, a method is proposed in JP Kokai (Laid-Open Unexamined Japanese Patent Application) S61-150760 which comprises disposing the guide rolls at non-uniform positions which are asymmetrical on the right and left.
However, this method may allow the occurrence of periodic molten steel level fluctuations within a mold as caused by slab bulging particularly when thin slabs having a thickness of 50 to 120 mm are cast at a high speed of about 3 to 5 m/min. Under the current situation of actual operation, the casting speed is slowed down to thereby suppress the occurrence of molten steel level fluctuations in cases where the periodic molten steel level fluctuations become intense and make it difficult to continue the operation.
In recent- years, such a continuous casting method for producing slabs having a thin thickness of 50 to 120 mm at a speed of about 3 to 5 m/min has been the target of development efforts from the viewpoint of reducing the cost of construction of associated equipment and reducing the personnel Under such circumstances, a method has been put to practical use according to which the continuous casting of thin slabs is combined with the rolling on a simple hot roll machine disposed on the continued casting line.
It is an object of the present invention to provide a machine for continuous casting of steel and a method of continuous casting of steel which prevent the occurrence of periodic molten steel level fluctuations in a mold as caused by slab bulging and make it possible to obtain good-quality slabs which have no defects in the slab surface as may be caused by mold powder trapping and no longitudinal cracks on the slab surface.
It is known that periodical molten steel level fluctuations in mold as caused by slab bulging occur in ordinary slabs about 250 mm in thickness. In the casting process, which have recently been put to practical use, for producing thin slabs having a thickness of 50 to 120 mm at a high speed of 3 to 5 m/min, periodical molten steel level fluctuations tend to occur more easily.
When thin slabs having a thickness of 50 to 120 mm are cast at a high speed of 3 to 5 m/min. the slab, just after withdrawal from the mold, has a thin solidifying shell and a thick liquid core. Therefore, slab bulging is apt to occur and, furthermore, since the sectional area of the mold is small, the molten steel level in the mold is readily influenced by the flow of liquid core molten steel flowing from the upstream side into the bulged portion, and by the flow of molten steel pressed back from the bulged portion to the upstream side.
Machine and method for continuous casting of steel of the present invention are a machine and a method intended to prevent periodic molten steel level fluctuations in a mold, as is caused by slab bulging.
The machine for continuous casting of steel of the invention is a machine in which, among the slab-supporting guide roll pairs disposed in the region in which the slab withdrawn from the mold contains a liquid core therewithin, at least four consecutive guide roll pairs are disposed under such conditions that the roll axis-to-axis distances (distances between two points in the central axis of the rolls) on the fixed side and loose side of the guide roll pairs and the roll halfway-to-halfway distances (distances between halfway of the rolls) on the fixed side and loose side, as determined in the direction of casting, are all different from one another.
By using this continuous casting machine, it is possible to prevent the occurrence of periodic molten steel level fluctuations in the mold as is caused by slab bulging and obtain good-quality slabs which have no slab surface defects caused by mold powder trapping and no longitudinal cracks.
The method for continuous casting of steel of the invention is a method which comprises carrying out the casting under such conditions that, among the slab-supporting guide roll pairs disposed in the region in which the slab withdrawn from the mold contains a liquid core therewithin, at least four consecutive guide rolls pairs are disposed so that the roll axis-to-axis distances on the fixed side and loose side and the halfway-to-halfway distances on the fixed side and loose side, as determined in the direction of casting, are all different from one another.
By applying this continuous casting method, it is possible to prevent the occurrence of periodical molten steel level fluctuations in mold as caused by slab bulging and obtain good-quality slabs without the occurrence of slab surface defects caused by mold powder trapping and longitudinal cracks.